president obama addressing the nation again earlier today on the boston marathon bombing. and he promised justice. >> it will take time to follow every lead and determine what happened. but we will find out. we will find whoever harmed our citizens and we will bring them to justice. >> scott cohn is leiv right now near the crime scene in the back bay section of boston. scott, what can you tell us? >> maria, with the next news briefing about 25 minutes away now, we can give you a little bit of news. first of all, nbc news is reporting that some fragments of the bomb or bombs are already at an fbi atf analytical center in quantico, virginia, where they ultimately hope to reconstruct both devices, or as much of them as we can. and while there are some grievous injuries here, the head of trauma at mass general says now he is much more confident this afternoon than he was this morning that there will not be further loss of life. three people, though, are dead. the final blocks of the marathon, going down boylston street are, according to people who have run this race, an incred